The present invention relates generally to methods for processing titanium alloys in the fabrication of powder metallurgy (PM) titanium alloy articles, and more particularly to a method for producing alpha phase titanium alloy PM articles with high resistance to loading and creep at elevated temperature.
Background information on the processing of titanium articles fabricated by PM techniques is fairly represented by or described in U.S. Pat. No. 4,534,808, U.S. Pat. No. 4,536,234 and U.S. Pat. No. 4,714,587, to Eylon et al, all assigned to the assignee hereof, and the publications, "Status of Titanium Powder Metallurgy", by Eylon et al (in Industrial Applications of Titanium and Zirconium: Third Conference, ASTM STP 830, pp 48-65 (1984)), "Developments in Titanium Powder Metallurgy", by Froes et al (J Metals 32: 2, 47-54 (Febuary 1980)), "Powder Metallurgy of Light Metal Alloys for Demanding Applications", by Froes et al (J Metals 36: 1, 14-28 (January 1984)), and "HIP Compaction of Titanium Alloy Powders at High Pressure and Low Temperature", by Eylon et al (Metal Powder Report 41: 4 (April 1986)). Teachings of these references and background material presented therein are incorporated herein by reference.
Titanium alloys are characterized by substantial room temperature strength resulting ordinarily from the presence of solid solution alloying elements such as aluminum, vanadium, zirconium and molybdenum, and from the presence in the alloys of one or both of the alpha and beta phases.
Titanium alloys composed primarily of the alpha phase have high temperature strength and resistance to creep significantly greater than that of alloys having appreciable beta phase content, because of higher temperature deformation resistance and limited slip systems of the hexagonal close packed (HCP) structure which characterizes the alpha phase. However, most elements used for solid solution strengthening, such as vanadium, molybdenum and zirconium, produce some beta phase at room and higher temperature. Presence of beta phase reduces resistance of the alloy to deformation (creep), particularly at high temperature, as a result of the body centered cubic (BCC) beta phase structure typically exhibiting numerous slip systems at high temperature. Alloys for use at high temperature are therefore formulated to include minimal beta phase and are known as alpha or near-alpha alloys. Reducing the beta phase content in an alloy conventionally requires concurrently reducing the content of desirable solid solution alloying elements. A desirable alloy would be one rich in strengthening alloying constituents but substantially free of beta phase.
The invention solves or substantially reduces in critical importance problems with existing PM techniques for fabricating titanium alloy articles by providing a method for producing alpha and alpha-rich titanium alloy PM articles with substantially improved resistance to loading at elevated temperature. According to the invention, application of very high pressure on titanium alloy powder containing alpha and beta phases, at a temperature slightly below the beta transus temperature of the alloy, followed by slow cooling of the powder, provides a powder compact of the alloy as a PM article virtually free of beta phase.
It is therefore a principal object of the invention to provide a method for processing titanium alloys in the fabrication of PM titanium alloy articles.
It is yet another object of the invention to provide a method for producing titanium alloy PM articles having improved high temperature deformation resistance.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.